One example of a manner in which agricultural run-off water can become contaminated is from the excessive use of fertilizers, particularly chemical fertilizers.
Ideally, a farmer wishes to apply just the right amount of chemical fertilizer: if he applies too little, plant protein production will be affected; if he applies too much, the economy of the operation will be affected. There is, however, a considerable margin between applying a little more fertilizer than the plant can usefully assimilate, and applying so much fertilizer that the plants would actually be poisoned. Economically, the farmer knows that it is far better to err on the side of a little too much fertilizer, than not quite enough.
Slow-release chemical fertilizers therefore tend to be over-applied, with the result that agricultural run-off water can contain substantial quantities of nitrate.
Nitrate pollution of groundwater can also arise from agriculture in the absence of chemical fertilizers; for example, some types of crop cultivation tend naturally to leave excessive nitrate concentrations in the soil, which leaches out into the run-off water. During the non-growing season, nitrogen from plants can become oxidised to nitrate.
Nitrate is a mobile and persistent anion in many groundwater flow systems. It is an especial problem that nitrate can remain intact for long periods in unconfined, shallow, aerobic, sandy, aquifers; when such aquifers are used for drinking water supply, it is a growing problem that the nitrate can exceed the drinking water contamination limit.
One of the difficulties in the effective treatment of the worsening nitrate problem lies in the fact that the nitrate is so diffuse. Any measure aimed at treating the nitrate in the water which it is contaminating, whether this is groundwater in its native aquifer, or water flowing in a river or stream, requires the handling of enormous quantities of water.
It is recognized that the problem might be alleviated to a great extent if the water could be treated actually at the agricultural site, before the nitrate enters the groundwater or other body of water.
It is an aim of the invention to provide a treatment which will break down the nitrate in water washed off an agricultural field, so that when the run-off water passes away from the field the water is substantially nitrate-free.
When a field is naturally well-drained, precipitation water may sink down vertically through the soil under virtually all conditions. The invention is mainly concerned with the treatment of pollution from fields that have drainage ditches and the like. The provision of drainage ditches, to cope with excess run-off water, is commonplace on farms. The use of land-drains, consisting of pipes of porous material inserted a few inches or more below the ground surface, is also common. Excess or storm water collects in the pipes and flows to a collection sump or ditch. Although powered pumping is sometimes utilised, land-drains are generally laid on an incline, whereby the collected water flows naturally to the collection sump, which is situated at a low point of the field. Land-drains may criss-cross a field, and may feed into a number of drainage ditches and collection sumps.
The pollution with which the invention is mainly concerned is that contained in the water that enters drainage ditches, land-drains, and run-off collection sumps.